1. Field of the Invention
The present invention relates to semiconductor integrated circuits, and more particularly, to an apparatus and method for using anti-fuse bond pads used to provide trimmed resistor values to the input terminals of circuits on an integrated circuit die.
2. Background of the Invention
Computer Automated Design (CAD) tools are now commonly used in the design of both analog and digital integrated circuits. CAD tools allow a designer to develop a circuit design using the software. Once the design has been developed, the software allows the design to be modeled and simulated. With digital circuitry, the software modeling and simulation is relatively simple. In the digital domain, transistors operate as switches. Since the on/off switching characteristics of transistors is straightforward to predict, the modeling and simulation of digital circuitry is highly predictable and accurate. With analog circuitry, however, the transistors operate in the linear range. As a result, the behavior of the transistors is far more difficult to predict due to a host of variables, such as variations in process technology, the differences in the gain of the transistors, and noise for example. It is therefore difficult to model and simulate analog circuitry with a high degree of accuracy.
As a consequence, analog designers operate within certain acceptable variances. These variances are commonly known in the semiconductor industry as “device corner distribution models”. Modern CAD tools used for analog typically use these distribution models. If the CAD software for a particular design shows that the circuitry is operating outside of an acceptable device corner (i.e., a distribution model), then either: (i) the circuit needs to be redesigned, (ii) process parameters need to be altered; or (iii) or a combination of both need to be performed to bring the circuit operation within the requisite operational parameters.
The industry trend requires higher levels of precision with analog circuitry. As a consequence, considerable focus is now being placed in the area of more accurate modeling in the semiconductor industry. With more accurate modeling, improved performance metrics and tighter integrated circuit performance can be achieved. Trimming is one technique that is used to achieve tighter specifications.
Trimming is a technique that is commonly used, particularly with analog circuitry, to bring a device that operates outside of specification back within specification. For example, a resistive network may be connected to a differential amplifier. The resistor network ideally includes two resistors of equal value connected to each input of the amplifier respectively. Due process and other variations, however, the value of the two resistors may not be the same (i.e., the circuit may be out of specification). Consequently, one or more trimming resistors may be provided in a resistive network with the two input resistors respectively. By selectively trimming the resistors in each network, the effective resistive value provided to the two inputs of the amplifier can be selectively made equal.
There are a number of well known trimming techniques commonly practiced in the semiconductor industry. With laser trimming for example, a thin metal film may be provided as part of a resistive network. By either selectively removing a portion or all of the thin metal film, the resistance of the resistive network can be altered. In another trimming method, a resistive network including one or more metal film or poly-silicon fuses is provided on the semiconductor chip surface. Again, by selectively blowing one or more of the fuses, the resistance of the network can be controlled. In different variations, the fuses can be either blown by ablation using a laser or by applying a high voltage. In another trimming technique, a transistor can be used to trim a resistor. The resistor, typically made of poly-silicon, is coupled between the transistor and ground. By rapidly turning the transistor on and off, current is driven through the poly-silicon resistor. The current can be used to either trim or blow the resistor. In yet another trimming example, anti-fuses can be used. For example, a resistive network of anti-fuses can be fabricated on the integrated circuit, where each of the anti-fuses is a transistor. By selectively applying a large voltage to the transistors of the resistive network, the gates can be selectively blown, causing the gate oxides to breakdown. As a result, the source and drain of each blown transistor are in electrical connection with one another. In this manner, the resistively of the network can be selectively trimmed.
There are a number of problems associated with current trimming techniques. The formation of poly fuses, thin film resistors, transistors and the like using to make fuses for trimming can represent a significant percentage of the overall surface area on a die. This overhead tends to make the die larger than it would otherwise need to be, decreasing yields. The characteristics of the above listed trimming elements also need to be incorporated in to the CAD software, making circuit performance modeling difficult to predict. Lastly, the aforementioned trimming procedures are highly labor intensive. Typically the chip has to be probed to determine resistor values. Then if necessary, the chips have to be individually trimmed using one of the aforementioned procedures.
An apparatus and method for using anti-fuse bond pads used to provide trimmed resistor values to the input terminals of circuits on an integrated circuit die is therefore needed.